The security of many systems, such as cryptographic systems, relies on unpredictability and irreproducibility of digital key-streams that are used for encryption and/or signing of confidential information. There are at least two fundamentally different strategies for generating random bits. One strategy is to compute numbers deterministically using, for example, a software-implemented algorithm that requires an initial input seed value. This class of random number generators is known as Pseudo/Deterministic Random Number Generators (PRNGs/DRNG). PRNGs must be provided with a random initialization value (seed) to produce an output value that resembles a random bit-stream. PRNGs are periodic, although the periods are typically very long.
Another strategy is to produce bits non-deterministically, where every bit of output is based on a physical process that is unpredictable. Such physical processes may be based, for example, on the production of thermal (resistance or shot) noise. This class of Random Number Generators (RNGs) is commonly known as True Random Number Generators (TRNGs). In hybrid RNG implementations, often a TRNG is used to initialize the PRNG (e.g., provide the seed for the PRNG).